Why the Greenwich Meridian Isn’t Where it Used to Be

Visitors to the “Prime Meridian of the World” at the Royal Observatory in Greenwich are often surprised that their GPS receivers display a longitude of 00° 00’ 05.3’’ W while standing over the meridian line in the pavement outside the Airy Transit Circle telescope. Airy’s Transit was installed in 1850 on the north-south line, and signals the start of the Universal day for the whole world. But now, one must walk east approximately 102 meters before navigation devices indicate zero. Most accounts of this longitude offset do not explain the apparent discrepancy, or else offer incomplete or incorrect explanations. A paper recently published in the Journal of Geodesy addresses the question frequently asked by London tourists with GPS receivers: “Why isn’t the Greenwich Observatory at zero longitude?”

The scientific reasons for the offset are presented by the research team of Stephen Malys and Nikolaos K. Pavlis of the National Geospatial-Intelligence Agency (NGA), John H. Seago of Analytical Graphics, Inc. (AGI), P. Kenneth Seidelmann with the University of Virginia (UVa), and George H. Kaplan with the U.S. Naval Observatory (USNO). They conclude that a slight deflection in the natural direction of gravity at Greenwich is mostly responsible for the offset, along with the way astronomical time was maintained over the 20th century.
A century ago, time of day was measured by astronomical instruments like the Airy Transit Circle. These optical devices measured “up” and “down” with reflective basins of mercury to establish a vertical plane of reference, or “meridian plane”. As the Earth turned, time of day was precisely measured with the telescope when certain stars crossed this imaginary plane. However, this meridian plane was aligned to the local direction of gravity instead of the center of the Earth. By the 1980s, space-age techniques, like satellite-navigation receivers, satellite-laser ranging, and Very Long Baseline Interferometry, were being used measure Earth rotation and these methods assumed that meridians of longitude passed through the center of the Earth. In 1984, the Bureau International de l'Heure (BIH, International Bureau of Time) fully converted from using a system of “astronomical” station coordinates based on local gravity, to a “geocentric” system of station coordinates based on distances from the center of mass of the Earth. It was this latter BIH system that provided the initial basis for the World Geodetic System 1984 (WGS 84), to which GPS coordinates are now referenced.
The research shows that the 102-meter offset can be attributed to the difference between two conventions for establishing latitude and longitude coordinates: astronomical versus geodetic. Their difference is known as “deflection of the vertical”, and high-resolution maps of Earth’s gravity confirm that the east-west component of the deflection is of the proper sign and magnitude at Greenwich to support this interpretation. It means that the meridian plane established by the direction of gravity beneath Airy’s Transit must relocate about 100 meters to the east if that vertical plane is to pass through the center of the Earth and continue yielding the same astronomical time as the old telescope.
To support their conclusion, the authors analyzed the differences between astronomical and geodetic coordinates of many other observatories that also contributed to the measurement of astronomical time during the twentieth century. Their analysis affirmed that there has been no significant world-wide rotation of longitudes between the former system originally based on the Airy Transit Circle, and the current system now used by GPS. Instead, the differences in the coordinates of old astronomical stations are simply a change of convention due to the localized deflection of the vertical. These conventional differences are not new to geodesists (who study the Earth’s size, shape, motion, and gravity), but only recently have global gravity models been precise enough to confidently attribute most of the observed offset at Greenwich to gravitational deflection.